Mercury switch



Julie 11, 1946. r c. H. LARSON 2,401,763

MERCURY SWITCH Filed Oct. 16, 1943 IN V EN TOR.

Carl M [Jar on Patented June 11, 1946 MERCURY SWITCH Carl H. Larson, Elkhart, Ind., assignor to The Adlake Company, a corporation of Illinois Application October 16, 1943, Serial No. 506,494

13 Claims. 1

Mercury switches are commonly required for various installations subject to excessive vibration, exceedingly rapid acceleration and deceleration, or severe mechanical shock from impact or other causes. Such forces and the inertia of the switch parts create severe stress in the switch structure and'sometimes seriously shorten the service life of a switch.

Metal envelope switches are particularly suitable to such difflcult service conditions, due to their rugged construction, but even so, the interior structure of the switch is sometimes dam aged when the service conditions are too severe.

In a displacement switch of the type in which the present invention has special utility, elec-- trodes or other members extend inwardly from the switch envelope as cantilever structures and may break or bend permanently under severe lateral mechanical shock especially if the cantilever structures are weighted at their free ends. Thus a simple lightweight rod-like electrode surrounded by glass, ceramic or other insulating material will withstand, to a required degree, vibrations and lateral mechanical shock without breaking or cracking but may sometimes fail if burdened by carrying at its free end substantial structure for the time delay functions or structure for other purposes.

One object of the present invention is to provide a sturdy switch construction. for long life under such severe service conditions as to impose excessive structural stresses. It is contemplated that the new construction will not only withstand the stresses incidental to the operation of various vehicles and heavy-duty machinery but will also stand up under such abuse as striking or dropping the switch itself. In this regard I have the concept of making one of the two electrodes of a mercury switch relatively large in cross section, preferably of open or hollow construction to straddle or enclose the other of the two electrodes in spaced relation thereto.

Such an electrode of large cross dimension has inherent structural efllciency to withstand excessive lateral stresses and may be loaded with any required switch structure without risk of failure. In some practices, the inner electrode is unburdened or at least is so lightly loaded with additional structure as to withstand all contemplated stresses without any special support. Other practices of the invention, however, are

inner electrodeis braced by the sturdy outer elecmum wastage and, moreover, will function in a predetermined desirable manner throughout the characterized by the concept of mechanically life of the switch.

Heretofore, time delay by mercury leakage has been achieved by providing a metering orifice in some insulating material such as glass or a ceramic. The nature of these materials and the manufacturing methods dictated by the character of these non-conducting materials have precluded precision and uniformity in the dimensions of the orifices. In some such switches the current controlled by the switch must be restricted to the small mercury stream at the orifice and the resulting high local resistance causes such heating as to burn the edges'of the orifice. The intense local heating is undesirable not only for its desructive effect on the material around the orifice but also because of its effect on the mercury fill.

My solution for the problem of producing precision orifices and of avoiding local heating is to achieve a switch combination that makes it possible to form the orifice in a metal wall. Metering orifices can be drilled in metal to exceedingly accurate dimensions and no localized high resistance to current flow can occur at an orifice in a metal wall since current can flow directly through the metal between mercury masses on opposed sides of the wall.

A third problem met with complete success in the preferred practices of my invention is to provide a displacement-type mercury switch of predominantly metal construction-that will carry high-amperage loads without excessive heating of the mercury and without current flow to any damaging extent along undesirable paths through the switch structure.

This problem is especially troublesome if the displacement means is a magnetically actuated armature guided by the metal walls of the switch envelope, since the switch envelope is connected to one side of the circuit and current tends to flow from the envelope through the armature and the mercury fill to the inner make-and-break zone of the switch. Sustained current flow substantially above 20 amperes will, in a conventional switch construction, burn the metal of the armature or even weld the armature to the switch envelope.

In the present invention a conductor or an electrode of substantially lower resistivity than mercury extends from the switch envelope to points lying between the armature and the makeandpbreak zone to provide a path oi substantially higher conductance than any Pam through the armature and to provide such a short mercury path as to minimise heating effect. of the current on the mercury illl.

Various switch installations require various operation characteristics with respect to time delay in closing and opening the switch circuit. One concept found in certain practiceso! my invention is that of employing interchangeable time elements or units (or mounting in the switch in a detachable manner at an advanced stage oi manufacture whereby nearly complete switch aubasaembiies may be stocked and then completed by the final addition of timing means selected in accord with a particular demand.

It will be apparent that all or the abovementioned features may be embodied in the construction of a switch electrode. That is to say, a

switch electrode may be of hollow construction and of substantial cross-sectional dimension to.

carry burdensome structure and yet withstand lateral stresses; the electrode may serve as a structural brace for an inner, less sturdy electrode: the electrode may provide the metal wall (or a metering oriiice; the electrode may extend between points of the armature and the make+ and-break acne to provide a relatively short mer.

om path tor current now; and, anally. the same electrode may incorporate a detachable time unit selected from a stock or various time units.

I110 above and other objects and advantages will be apparent in my following detailed description taken with the accompanying drawing.

In the drawing which is to be regarded as merely illustrative,

Fig. l is a longitudinal sectional view of one form of my invention;

.l'ls. 2 is a transverse section taken as indicated by the line 2-2 oi Fig.1; and

Fig. 3 is a longitudinal sectional view of a second form of the invention.

The form oi'the invention shown in Figs. i-

and 2 includes a cylindrical switch envelope ll incloeing a mercury nil ii and a displacement means generally designated II, the displacement means being in the form of an armature for electromagnetic switch actuation. In the normal or tie-energized state of the switch, the displacement means I! is free to float partially submerged in the mercury nil as shown in Fig. 1. Ilnergisation of an exterior electromagnetic means generally designated it draws the displacement means il downward thereby to raise the level oi the mercury fill in a well-known manner. When the liquid level of the mercury fill rises, a conducting bridge 01' mercury is formed between what may be termed an inner electrode generally designated i5 and an outer electrode generally designated ll.

The electromagnetic means I! may comprise nitrogen, argon and helium hydride.

4 envelope l0 and is suitably bonded thereto. Prelerably a bracket 25 is adiustably mounted on the iron circuit l8 by'a machine screw 26 to engage the rubber collar 22 as shown.

The switch envelope iii comprises a cylindrical wall 21, an upper cap 28 and a lower cap Ill. The cylindrical wall 21, which may be polished on its inner surface to minimize frictional movement of the displacement means II, should be made of metal that is both nonmagnetic and resistant to wetting by mercury. A suitable metal, for example, is a stainless steel known as "18-8 steel having besides iron, approximately 18% chromium and 8% nickel.

The upper cap it may be a metal disc or a pressed sheet metal member with an upturned flange ii to fit inside the cylindrical envelope wall 21. The joint between the cap 28, and the cylindrical wall 21 may be sealed in any suitable manner, for example, by welding as indicated at 32.

In mypreferredrconstruction, the upper cap 2| is provided with a. central bore 33 normally sealed by a headed pin or plug 35. In the course of manufacture air may be evacuated from the switch envelope through the bore 33 and then a suitableinert gas or mixture of inert gases may be introduced through the same bore to till the envelope under pressure. The gas or gases may be selected from a group comprising hydrogen, After the gas has been introduced to the desired pressure, the head of the pin or plug 35 is welded to seal the bore 33.

The lower cap 30 may likewise be a metal disc or pressed sheet metal member with a flange 3| to fit inside the cylindrical envelope wall 21 and the Joint may likewise be sealed by welding as indicated at 31. The cap 30 has a central aperture surrounded by a cylindrical flange 3! to embrace a body of insulating material 40. In the present construction, the insulating material ll is part of the structure of the inner electrode ii. the insulating material forming a tubular wall ll around an inner electrode member or electrode proper l2.

It is requisite that the materials of the cap Ill, insulating body iii, and the electrode member I! all have substantially the same coefficient oi thermal expansion to maintain eilective sealing relationships under varying temperature conditions. In my preferredembodiment of the invention the electrode body 42 is made of molybdenum, the insulating body lil is a hard glass such as Corning G705AJ fused both to the electrode member and to the cap Ill, and the cap is fabricated from a metal known to the trade as "Kovar," an alloy manufactured by the Stupakoi! Laboratories, Pittsburgh, Pennsylvania.

One wire 43 of the circuit to be controlled by the switch may be soldered to the cap Iii as shown or any place on the switch envelope, and the other wire 45 of the circuit may be soldered to the external end of the electrode member 42.

The tubular glass insulating wall 4! terminates short oi the upper end of the electrode member 42 so that the electrode member has an end portion exposed for mercury contact in the makeand-break zone of the switch between the minimum and maximum levels of the mercury fill. It

is desirable that this end portion of the electrode the material employed in fabricating such a cup be efllciently bonded to the tubular glass wall II and have the same coeincient of thermal expansion. The cup may be made, for example, of a ceramic known as Alundum manufactured by the Norton Company, Worcester, Massachusetts and may be simply fused to the upper endof .the tubular wall 4!, in accordance with the teachings of my Reissue Patent #.2l,661 issued December 17, 1940.

In designing the outer: electrode Hi the first consideration with respect to meeting destructive forces is to form the outer electrode with a relatively-wide base for anchorage to the metal cap 30 and to make the electrode relatively wide in cross section. To this end, the outer electrode may be of open or frame construction or may be of hollow construction. In the present form of the invention, the outer electrode'isof hollow or closed construction and preferably ismylindricai in configuration so that it may meet forces from all lateral directions with equal efficiency.

If the switch is to operate-with time delay the required time delay means may be carried by the structurally efllcient outer electrode without risk of overburdening the electrode. time delay means provides a, fluid passagefor restricted gas flow into or'out' of a gas trap and/or a fluid passage or'mercury leak orifice for controlling the rate of mercury flow to' or from the make-or-break zone within the configuration of The inner electrode l5 constructed as described, and only lightly burdened by the small ceramic cup 46, will. withstand, without damage relatively severe lateral mechanical forces encountered in many installations. 'Io meet exceptionally destructive lateral forces, however, it is desirable to reinforce or embrace-the inner electrode and be accomplished by forming the tubular insulating wall 4| with a circumferential bead 56 to support a reinforcing collar orspider 51. As

., indicated in Figs. 1 and 2, the spider'5'l may be Commonly, the

the outer electrode. In this first form of my inventiorr restricted gas flow determines the extent the previously mentioned stainless ste'elorI may use plain or chromium plated cold rolled steel. For high amperage loads, it may be desirable to use a metal such as copper having substantially lower resistivity than iron or steel thereby highly favoring current flow to the .make-and-break zone through the tubular electrode member 41 rather than through the displacement means 12.

The desired gas trap may be'formed by or in the tubular electrode member 41 or may be mounted on the electrode member. Fig. 1 shows a gas trap in the form of a' bell-shaped member cut away to provide relatively large mercury ports 5| with upper edges 52, which upper edges are preferably substantially horizontal. Any expedient may be employed for mounting the gas trap 50 on the tubular electrode member 41 but in my preferred construction I simply thread the gas trap onto the upper end of the electrode member as shown inFig. 1. The material employed for the gas trap may be the same metal as the material of the tubular electrode 41;

The passage means for restricted gas flow from for example, the previously mentloned ceramic,

material known as Alundum which is availablej'i'n several grades including dense, medium and coarse.

in the form of a ring of ceramic material or glassdimensioned to fit snugly against the surrounding tubular electrode member 41 and cut away on its periphery toprovide passages 58 for free mercury flow. In effect, the spider 51 structurally unites the outer electrode with the inner electrode to give the inner electrode the benefit of strength inherentin the tubular configuration of the outer electrode.

The displacement means or armature I! may comprise a cylinder 60 of suitable ferromagnetic material such as iron and may be adapted for guidance by'the surrounding switch envelope. In the present construction, for example, the cylinder 60 is formed with integral radial flanges 6| at its'top and bottom, each of the flanges being cut away as shown in Fig. 2 to provide guiding lugs 62 and intervening mercury flow pas-.

sages 63.

Preferably, I employ coil springs 65 of nonmagnetic material at the opposite ends. of the ferromagnetic cylinder iii 1 "to serve as buffer means. If either of the caps 28 and 30 is of ferromagnetic material, such coil springs are especially desirable to prevent the displacement means [2 from clinging to the cap magnetically. The term displacement means"- is to be understood as applying to the ferromagnetic cylinder 60 or as covering the cylinder together with the'coil springs 65.

In some practices of my invention such as the practices illustrated in the present disclosure, I

place a wall of insulating material such as glass' or some suitable ceramic between the cup 61nd the metal of the displacement means l2, an important purpose of the insulating wall being to lengthen the minimum possible path of current flow through mercury from the displacement' meansto the mercury pool 49 in the cup. Any

' current flowing between the metal of the disferromagnetic "cylinder 60.- In my present'construction, the insulating wall is in the form of a cylinder or tube 66 mounted inside the ferro-' magneticcylinder'ili in the manner of a liner.

Preferabl'yfthe insulating cylinder 66 extends beyond the opposite ends of the ferromagnetic cylinder 60 but terminates short of the unrestrained extent of the two coil springs 65. The

coil springs may tightly embrace" the ends of the insulating/cylinder6G as"means for holding the insulating cylinder'66 assembled to the surrounding ferromagnetic cylinder 60.

In the preferred practice of my invention another expedi'ent'employed to prevent current flow between the switch envelope and the displace- 7 ment means is the coating of the displacement means with a suitable high resistance material selected from materials that will not react with the mercury. For example, I may coat at least the peripheral surface the displacement means with oxide of iron as stated in my Patent 2,121,-

how through the switch because the major mercury body isfseparated from the mercury pool 9. Energization oi the electromagnet draws the displacement means I2 to its lower positionthere+ by displacing the mercury iill to a maximum mercury level, for example, the maximum level indicated by dotted lines 68. The maximum mercury level isoi course substantially above the level oflthemercury cup 48 and the suddenrise oi the mercury outside oi the tubular electrode to, the maximum" level 68 causes the mercury inside the gas trap ill to rise to an initial level slightly above the horizontal port edges 52, gas

in the trap being compressed by hydrostatic pressure derived from the mercury column outside the trap. The resulting pressure difierential. between.- the gas in the trap and the gas above the trap causes gas to now upward out oi thetrap at a retarded rate governed by the now capacity. of the porous plug 63. Eventually the gas volume inside the trap diminishes sumciently to permit the mercury column inside thetrap to rise above the lip of the cup 46 and merge with the mercury pool 49 to close the switch 'circuit.

Upon subsequent de-energiz ation of themelectromagnet, the displacement means rises immediately to its original floating position with simultaneous drop of the mercury level outside the trap. As soon however, as the mercury level outside the trap drops below the horizontal port edges I, gas. is free to enter the gas trap laterally and the mercury column inside the trap thereupon collapses suddenly to open the switch circuit in an instantaneous manner.

Fromthe manufacturing standpoint, an im-' portant feature of the described construction is hat subassemblies may be fabricated and stocked,

be completed as demand arises and in accord with time delay characteristics dictated by the demand. For example, such a subassembly may comprise the cylindrical wall 21 of the envelope, the lower cap 36 welded thereto and the completed inner electrode I! carried by the lower cap ID. The only internal switch structure missing iromithe subassembly would .be either the gas trap ill alone or the gas trap together with the tubular electrode member 41. At the time of final assembly, a gas trap 50 would be selected from a diverse stock of gas trapsto provide the time delay action specified by the purchaser of the switch, the gas trap or the gas trap together with the tubular electrode being simply threaded onto the subassembly.

The second form of my invention shown in Fig. 3 is largely similar to the first described form, as indicated by the use of corresponding numerals to designate corresponding parts. In this second form of the invention, I employ an outer electrode III that consists of a simple metal tube provided .with a mercury leak orifice 12 below the; normal level 13 of the mercury iill. mine particular construction shown in Fig. 3, the upper end of the tubular electrode 10 is cut at a slant to provide an overflow lip 15 below the maximum level of the mercury fill.

Upon energization of the electromagnetic means l3, the displacement means I! is drawn downward to cause the mercury level to rise above the lip 15 whereupon the mercury flows into the outer electrode 10 to merge with the mercury pool 49 and thereby close the switch circuit.

Upon subsequent energization of the electromagnetic means l3 to permit the displacement means to return to the original position shown in Fig. 3, the mercury level outside the electrode 10 drops initially below the normal level 13 but mercury within the electrode III is temporarily trapped and can drop in level only at a retarded rate determined by the area of the orifice I2 and the differential in mercury levels. Eventually the mercury inside the electrode I0 drops below the lip of the mercury cup 46 to open the circuit.

The'invention obviously may be employed in slow. and fast acting switches within the teachings of this disclosure.

In switches constructed as described herein, the outer electrodes are hollow but being of relatively large diameter may nevertheless have relatively large cross-sectional area and therefore the capacity to carry considerable amperage. The wall of a-hollow electrode may be relatively thick if desired.

It is to be noted especially that the metal wall of the electrode lies between the surrounding displacement means and the central cup. The significance of this relationship may be appreciated by comparing the normal path of current flow through the outer electrode with any path of current flow through the displacement means. The outer electrode provides-an all-metal path for current flow from the switch envelope to the level of the cup leaving only the short radial distance between the electrode and the cup for current flow throughmercury. In contrast, the displacement means is not connected with the metal envelope to form a continuous metal path and, moreover, all mercury paths from the displacement means to the central cup are excessively long.

It is apparent that even under high amperage loads no damaging current will flow from the switch envelope to the armature. In effect, the outer hollow electrode is a conductor shield between the cylindrical wall of the switch envelope and the mercury cup, the conductor shield being necessarily at the same instant potential as the cylindrical wall of the envelope to preclude current flow radially between the'envelope and the mercury pool in the cup.

Selected embodiments of the invention have beendescribed in specific detail herein for the purpose of illustration and to teach the principles involved. Various changes and substitutions may be made under my basic concepts and therefore the following claims are to be construed as broadly as the prior art will permit.

I claim:

1. In a mercury switch, a metal switch envelope for connection with one side of a switch circuit, a mercury fill, a metal displacement means of hollow configuration in said envelope, means to shift said displacement means thereby to cause the mercury level to rise and fall in a make-andbreak zone inside said displacement means to close and open the switch circuit, and a metal conductor inside said displacement means spaced therefrom, said metal conductor extending around said make-and-break zone and being connected with said envelope to have the same instant potential as the envelope for preventing damaging current flow between the envelope and the displacement means.

2. In a mercury switch, a swich envelope, a mercury fill, displacement means movable to vary the liquid level of the mercury fill, a first electrode in said envelope exposed for mercury contact above the minimum liquid level of the mercury fill, a second electrode extending inward at least approximately as far as the inner end of the first electrode, the said second electrode being of relatively large cross section to withstand lateral mechanical shocks, said second electrode enclosing a space for mercury extending on opposite sides of said first electrode, and means extending across said space at a level substantially above the bottom of the space to mechanically interconnect the two electrodes to brace th first electrode against lateral mechanical shocks, said interconnecting means permitting fluid flow through its level from above and below.

3. In a mercury switch, a switch envelope, a mercury fill, displacement means movable to vary the liquid level of the mercury fill, a first electrode in said envelope exposed for mercury contact above the minimum liquid level of the mercury fill, a second electrode extending inward at least approximately as far as the inner end of the first electrode, the said second electrode being of relatively large cross section to withstand lateral mechanical shocks, said second electrode defining a space for mercury surrounding said first electrode, there being restricted fiuid communication between said space and the exterior of said second electrode to retard changes in the mercury level in the space in response to changes in the mercury level outside the secondelectrode, and means extending across said space at a level substantially above the bottom of the space to mechanically interconnect the two electrodes to brace the first electrode against lateral mechanical shocks, said interconnecting means permitting fiuid fiow through its level from above and below. I a

4. In a mercury switch; a switch envelope, a body of mercury partially filling the envelope, a body of gas above the mercury, displacement means movable to vary the liquid level of the mercury, a first electrode in said envelope exposed for mercury contact above the minimum liquid level of the mercury, and a second hollow electrode defining a space surrounding said first electrode above the minimum liquid level of the mercury, there being sufficiently restricted fluid space and the exterior of the second electrode for rapid mercuryffiow into and out of the space at said lower portion.

5. In a mercury switch, a switch envelope, a body of mercury partially filling the envelope, a body of gas above the mercury, displacement means movable to vary the liquid level of the mercury, a first electrode in said envelope exposed for mercury contact above the minimum liquid level of the mercury, and a second hollow electrode surrounding said first electrode and defining a vertically extensive space extending above and below the minimum liquid level ofthe mercury, there being relatively free fiuid communication between said space and the exterior of said second electrode at an intermediate level above the minimum mercury level and at a lower level to cause the mercury level in the space to rise rapidly to said intermediate level in response to rise in the mercury level outside the second electrode, there being sufficiently restricted fluid communication between said space and the exterior of the second electrode at a level substantially above the minimum mercury level for sufliciently restricted gas flow out of the space to retard rise in mercury level from said intermediate level in response to rise in the mercury level outside the second electrode.

6. In a mercury switch, a switch envelope, 9. body of mercury partially filling the envelope, a body of gas above the mercury, displacement means movable to vary the liquid level of the mercury, a first electrode in said envelope exposed for mercury contact above the minimum liquid level of the mercury, and a second electrode having a wall formin a gas trap at the level at which mercury electrically bridges the two electrodes, said trap having a sufiiciently restricted gas vent to retard the escape of gas thereby to retard the rise of mercury in the trap to said bridging level, said trap being apertured at a lower level below said bridging level for the free infiow of gas and free outflow of mercury to cause the mercury column in the trap to collapse suddenly when the mercury outside the trap falls to said lower level.

7. In a mercury switch, a switch envelope, a mercury fill, displacement means movable to vary the liquid level of the mercury fill, a first electrode in said envelope exposed for mercury contact above the minimum liquid level of the mercury fill, a second electrode spaced from said first electrode, said second electrode extending on opposite sides of said first electrode to provide a relatively wide cross section for withstanding lateral mechanical shocks, and a gas trap supported at the level at which mercury electrically bridges the two contacts, said trap having a sufficiently restricted gas vent to retard the escape of gas thereby to retard the rise of mercury in the trap to said bridging level, said trap being apertured at a lower level below said bridging level for the free infiow of gas and freeoutfiow of mercury to cause the mercury column in the trap to collapse suddenly when'the mercury outside the trap falls to said lower level.

8. In a mercury switch, a switch envelope, a mercury fill, displacement means movable to vary the liquid level of the mercury fill, a first electrode in said envelope exposed for mercury contact above the minimum liquid level of the mercury fill, a second electrode spaced from said first electrode, said second electrode extending on opposite sides of said first electrode to provide a relatively wide cross section for withstanding lateral mechanical shocks, and a gas trap supported at the level at which mercury electrically bridges the two contacts, said trap having a sufficiently restricted gas vent to retard the escape of gas thereby to retard the rise of mercury in the trap to said bridging level, said trap being structurally separate from and attachable to said second electrode whereby in the course of manufeature 0! the switch. selection may be made from a variety of gas traps for various operating characteristics.

9. In a mercury switch. a switch envelope, a mercury fill, displacement means movable to vary the liquid level of the mercury fill, a first upright electrode in said envelope exposed for mercury contact above the minimum liquid level of the mercury fill, a second upright electrode extendlng upward at least as far as the upper end of the first electrode. said second electrode extendin: on opposite sides or said first electrode to provide a relatively wide cross section for withstanding lateral mechanical shocks, nomconductin: means mechanically interconnecting said electrodes near the bottom oi said envelope, and a non-conducting bridge spaced substantially above said non-conducting means mechanically interconnecting said two electrodes to brace the first electrode against lateral mechanical shocks.

10. In a mercury switch, a metal switch enve- 'lope for connection with one side of a switch circult. a mercury fill, a metal displacement means or hollow configuration in said envelope, said displacement means being shaped and dimensioned for guidance by the envelope. means to shiit said displacement means thereby to vary the liquid level oi the mercury fill, a first electrode for the other side of the circuit having a portion exposed for mercury contact between the minimum and maximum liquid levels of the mercury fill, and a second electrode 0! lower resistivity than mercury connected with said envelope and extendin: between said displacement mean and said exposed portion of the first electrode to provide a mercury path to the exposed portion substantial- 1y shorter than mercury paths to the exposed portion from any points on said displacement means whereby said second electrode cooperates with the mercury fill to provide a path from said envelope to said exposed portion of substantially greater conductance than any path through said displacement means, thereby preventing damaging current flow between the envelope and the displacement means under relatively high amp rase switch loadins.

11. In a mercury switch, a closed metal envelope including a base through which a relatively long glass insulated conductor is hermetically sealed at its lower end with its upper end exposed to serve as an electrode, a mercury fill, a magnetically actuated displacer for shifting the mercury fill into and out 01' contact with said exposed electrode, said glass insulated conductor, because of its length, bein relatively fragile and subject to breakage particularly when subjected to lateral shock, and a tubular metallic member rigidly fixed to the envelope base, telescoped over the said conductor, and limiting lateral movement of the upper end of the conductor to amounts which will not cause cracking or the glass on the said conductor.

12. In a mercury switch, a closed metal envelope including a base, an insulated conductor hermetically sealed through the base and having its upper end exposed to serve as an electrode, a mercury fill, a metal displacer.magnetically actuated to shift the mercury fill into and out of contact with the said electrode, a tubular metallic member having substantially greater conductivity than mercury rigidly fixed to the base, telescoped over the said conductor, and extending upwardly to a point relatively close to said exposed electrode, said envelope with its base, tubular metallic member and mercury fill being adapted to be electrically connected to one side 01' an electric circuit and said conductor to the other side, said tubular member, because of its proximity to said exposed electrode and because of its superior conductivity relative to mercury, substantially reducing the flow of current which takes place from the envelope to the exposed electrode through the displacer when the tubular metallic member is eliminated.

13. In a mercury switch, the combination of a switch envelope, a mercury fill, spaced electrodes in the envelope including an insulated conductor sealed through the base of the envelope and projecting upwardly through the mercury fill with the upper end of the conductor exposed to form one of said electrodes, a mercury displacer for shifting the mercury fill to make and break an electrical circuit through said electrodes, and a gas leakage time delay element over said exposed electrode, said time delay element being mounted on and supported solely by a tubular sleeve telescoped over and spaced from the insulated conductor, said sleeve being rigidly afilxed to the switch envelope.

' CARL H. LARSON. 

